Q.: I am designing a supported slab that will carry forklift traffic. I am planning to use lightweight concrete. What wearing surface would be appropriate with lightweight?

A.: ACI 213 "Guide for Structural Lightweight Aggregate Concrete" states that "abrasion resistance of structural lightweight concrete varies with compressive strength in a manner similar to normal weight concrete. Therefore, the abrasion resistance of all lightweight concrete may not be suitable for steel-wheeled or exceptionally heavy industrial traffic in commercial establishments. As the severity of wear becomes less, i.e., in light warehousing, markets, public buildings, schools, churches, and residences, the abrasion resistance should be as satisfactory as that of normal weight concrete."

ACI 213 goes on to note that the abrasion resistance of structural lightweight concrete can be improved by combining the lightweight coarse aggregate with a hard fine aggregate or by applying a natural sand-portland cement or iron-aggregate dry shake to the surface of the concrete. The cost of these methods to increase the surface durability, however, may offset the saving in framing to support the lighter weight concrete.

Rick Smith, with Structural Services, notes that there are other important issues with lift trucks on suspended floor slabs, floor stiffness, shear transfer to the supporting structure, and rotation of the slab over girders and beams. He typically designs the slab as a composite deck, since it will make the floor stiffer at a small increase in cost.